1. Technical Field
The present disclosure relates to technical fields of measurements of reflectance spectrum, and particularly, to a spectrophotometric colorimeter based on LED light source and a method for realizing the same.
2. Description of Related Art
Reflectance spectrum measurement is the basic way of color measurement, which calculates tristimulus values and other parameters of the color by obtaining the spectrum reflectance curve from the surface of the to-be-measured sample within the visible light range. Spectrophotometer is the predecessor of spectrophotometric color measurement instrument. Both the spectrophotometer and the spectrophotometric color measurement instrument are used to measure the spectrum reflectance or spectrum transmittance of the measured sample. Since various techniques about the spectrophotometer are relatively mature, the techniques are well applicable in different colorimeters. According to structures and measuring principles, the existing color measurement instruments are divided into three types:
photoelectric integrating color measurement instruments, spectral scanning color measurement instruments, and spectrophotometric colorimeters.
A photoelectric integrating color measurement instrument generally performs integral measurement of the whole visible wavelength using a single sensor without adopting light splitting principle.
A spectral scanning color measurement instrument has a structure similar to that of the spectrophotometer. This type of instrument generally adopts a 0: D geometrical condition. By rotating a grating, the instrument splits light emitted from a light source into monochromatic lights; the monochromatic lights are irradiated onto the measured sample, and then the reflection light signals are measured by a single sensor.
A spectrophotometric colorimeter does not split the light emitted from the light source. The light emitted from the light source is irradiated onto the measured sample. The reflection light from the sample is gathered and split, and is further detected by an array sensor. The spectrophotometric colorimeter can provide geometrical conditions including D:0 and 45:0. At present, most of portable color measurement instruments are spectrophotometric colorimeters.
Since color measurements have special applications and measurement features, following special considerations are required when spectrophotometric colorimeters are designed.
Firstly, spectral detection ranges of the spectrophotometric colorimeters are different. The spectral detection range of a regular spectrophotometric colorimeter mainly focuses on visible light. A spectral detection range recommended by CIE is 360-830 nm. Dividing the spectrum into a shortwave portion and a longwave portion has a small effect on the colorimetry calculation, thus, in the designs of most of color measurement instruments, the spectral detection range is set to be 380-780 nm. However, since measured samples are different, if the spectral detection ranges of the instruments are different, the measurement results may be greatly affected. Particularly, when material containing fluorescent matter is measured, whether the spectral distribution of the light source of the instrument includes UV light or not may greatly affect the measurement result.
Secondly, the shape of the reflectance spectrum from the surface of the object is generally relatively mild, and in the calculation of the tristimulus values of a color, compared with a spectrophotometer, requirement for a wavelength resolution of a spectrophotometric colorimeter is relatively low. The calculated wavelength resolution recommended by CIE is 1 nm. In applications, the wavelength resolution can be 10 nm. At present, the wavelength resolution chosen by most of spectrophotometric colorimeters is 10 nm.
Moreover, the measurement geometrical condition in the color measurement of the spectrophotometric colorimeter is much more complicate with respect to the spectrophotometer. CIE sets multiple kinds of geometrical conditions for the measurements of reflective samples and transflective samples. Different applications require different measurement geometrical conditions. Since the technical solutions of different device manufacturers are different from each other, design methods of the measurement geometrical conditions are correspondingly different. Different measurement geometrical conditions may cause differences among measurement results of these instruments. Thus, strict designs of measurement geometrical conditions are required in the designs of the instruments.
The measurement principle of the spectrophotometric colorimeter is as follows: the spectrophotometric colorimeter measures the spectrum reflectance or the spectrum transmittance of the measured object, selects the standard illuminant and the standard viewer from CIE, and obtains the tristimulus values of the color by integration. The spectrophotometric colorimeter is actually a physical colorimeter which measures the spectrum reflectance from the sample surface and calculates a series of psychological or physical parameters, including the tristimulus values X, Y, and Z of the color on the sample surface, according to the spectrum tristimulus function provided by the CIE standard color system. The spectrophotometric colorimeter mainly consists of a light source, a light splitting system, a photoelectric detecting system, an electrical control system, and a data processing system, etc.
A traditional spectrophotometric colorimeter generally adopts a tungsten halogen lamp or a xenon lamp as the test light source. The tungsten halogen lamp is the most commonly-used light source of the visible wavelength in the color measurement. Due to high stability, the tungsten halogen lamp is suitable to be used as an illumination light source or a radiometric calibration source in the color measurement. The most important characteristic of the tungsten halogen lamp is that the output spectrum line thereof is very smooth, without fractures, peaks, and depressions, as shown in FIG. 1. However, the energy of the spectrum distribution of the tungsten halogen lamp in the shortwave portion and the ultraviolet light portion is insufficient, which may cause two problems as follows. The first one is that the signal-noise ratio of the measurement signal of the shortwave portion is relatively low, which may affect the repeatability of the measurement; the second one is that the tungsten halogen lamp cannot provide ultraviolet light spectrum energy required in the measurement of the fluorescent material.
In addition, the power consumption of the tungsten halogen lamp is high, correspondingly shortening the working time of the instrument. If the tungsten halogen lamp is used as the test light source in the spectrophotometric colorimeter, optimizations should be considered aiming to the above situation. The xenon lamp has good spectrum energy distribution in the spectral ranges of visible light and ultraviolet light, as shown in FIG. 2. Many spectrophotometric colorimeters use pulse xenon lamps as illumination light sources. However, the pulse xenon lamp has high power consumption and thus has a shorter life.
With the development of the LED technology, more and more instrument manufacturers use LEDs as measuring light sources of portable spectrophotometric colorimeters. A LED light source has advantages including long life, rapid response, and low power consumption. In order to ensure that the illumination light source has sufficient spectrum distribution in the spectral range of visible light, multiple LED light sources are required to be combined to form a composite LED light source.
The State Bureau of Technical Supervision made JJG 867-1994 Verification Regulation of Spectrum Tester in 1994, and published JJG 595-20025 Verification Regulation of Colorimeter and Color Difference in 2002. The two verification regulations are respectively applied in the verifications of spectrum scanning color measurement instruments and photoelectric integrating color measurement instruments and respectively have specific test indicators. At present, no verification regulations are applied in spectrophotometric colorimeters. At present, in practical verifications, each metrological verification institution uses the verification regulation JJG 595-2002 to verify the spectrophotometric colorimeter. JJG 595-2002 provides requirements for the reproducibility detection of the colorimeter. The reproducibility detection is used for evaluating the uniformity of the illumination provided by the instrument on the surface of the measured sample. After the instrument is started and pre-heated, the instrument continuously measures a standard reference white plate eight times to verify the reproducibility. In each measurement, the instrument rotates about the white plate over 45 degrees. The indicator Δl of the reproducibility of the measurement result is calculated according to the following equation:Δl(u)=|uīū|max;wherein ui is the ith measurement value of each parameter (tristimulus values, chromaticity coordinates, color difference, etc.) of the instrument, and ū is the average value of the measurements.
The evaluation of the reproducibility of the instrument can be scaled as the following table:
ItemIndicatorScale oneScale twoReproducibilityΔl(Y)≦1.0≦2.0Δl(x), Δl(y)≦0.002 ≦0.006Δl(ΔE)≦0.5≦0.7
Taking the spectrophotometric colorimeter with the D:8 structure shown in FIG. 3 as an example, light emitted from the illumination light source is incident on the sample surface after becoming uniform in the integrating sphere. The reflection light from the sample surface enters the sensor (detector) 2. Ideally, the incident light uniformly irradiates the sample surface after becoming uniform in the integrating sphere, which is not relative to the position of dS; and the light intensity distribution of each spot on the sample surface satisfies the Lambert distribution. However, in practical applications, since the diffusion illumination environment provided by the integrating sphere is not ideal, the distribution of the light radiation power intensity or light intensity angle of the wavelength of the light source which is irradiated to the sample surface is not uniform. In this situation, when the sample with non-uniform surface color or texture rotates at the bottom thereof, through the reproducibility detection, the uniformity degree of the illumination provided by the illumination light source on the surface of the measured sample can be evaluated. However, in the metrological verification regulation, the used measured sample is the standard white plate. Since the surface color of the standard white plate is uniform and the surface texture of the standard white plate is also relatively uniform, the surface reflection spectrum is considered to be unchanged as the position of the sample changes. In this state, when the sample rotates at the bottom thereof, the target of the detection cannot be achieved.
Therefore, in the practical detection, measured samples with non-uniform surface colors need to be chosen to evaluate the reproducibility of the instrument. The samples as shown in FIG. 4 are designed, and then a couple of white and black semi-circles are printed on a common printing paper by a printer. The diameter of the circle is 10 mm.
The samples shown in FIG. 4 are tested by Minolta spectrophotometric colorimeter CM-700D and X-Rite spectrophotometric colorimeter SP-64, both of which have the D:8 structure. The tests are taken in the SCI condition. In each measurement, the center of the measured sample coincides at the center of the measurement caliber of the instrument, the sample is rotated over 45 degrees after each measurement, and total 8 measurements are performed to evaluate the reproducibility of the instrument. The measurement results are shown in FIG. 5 in which the reproducibility ΔL(Y) respectively reaches 4.88 and 4.96. The reproducibility cannot reach the verification requirement for the color measurement instrument set in JJG595-2002.
When the white plate in the BCRA series of color plates is used as the standard white plate to verify the reproducibility of the Minolta spectrophotometric colorimeter CM-700D and X-Rite spectrophotometric colorimeter SP-64, the test results are shown in FIG. 6. Since the surface of the white plate is relatively uniform, the reproducibility ΔL(Y) respectively reaches 0.62 and 0.56. From the measurement results shown in FIGS. 5 and 6, non-uniform samples can better detect the problem about the reproducibility of the instrument.
In technical solutions provided in prior art, optimization designs are not adopted aiming to the reproducibility of the instrument, which thus needs improvement.